Conductive materials which are, to some degree, plastic would be useful in various technological applications.
For example, the level of electromagnetic radiation in the environment is relatively high, due to the increased utilization of electronic equipment. Sensitive electronic devices need protection from this radiation.
Protection can be provided by metallic shields. However, the substantial weight of solid metal shields, as well as the relatively high costs of such shields, makes their use impractical. Moreover, housings for electronic components are often constructed in two or more parts to allow for ready access to the electronic components. Metallic shields are ineffective for sealing gaps between the parts of such housings, and therefore are not suited for protecting the components within from adverse atmospheric conditions. Accordingly, it is desirable to provide an electromagnetic interference (EMI) shield which is sufficiently plastic to act as a sealant, to protect housed electronic components from the elements as well as to protect the components from EMI.
The basic requisite for an EMI shielding material is that it conduct electricity. Electrical conductivity can be imparted to plastics via incorporation of conductive fillers into the plastic matrix. Typical conductive fillers contain silver, nickel or copper. However, the housings for electronic components are typically made of aluminum. Because silver, nickel and copper are more noble than aluminum, these metals will set up a galvanic cell in contact with aluminum in the presence of moisture. In other words, there is an electrochemical potential difference between aluminum and the conductive fillers. This results in accelerated corrosion of the aluminum housing, which is referred to as galvanic corrosion (1).
Another application is use of suitably conductive, plastic material for electrostatic dissipation. Such a plastic material can be deposited on, for instance, a metal surface so that, when a person carrying a static charge touches the coated surface, the charge is bled off by the conductive material, rather than discharged in a spark. Of course, the plasticity of the material is useful in conforming it to the surface's configuration, etc. It will be appreciated that the same galvanic corrosion difficulties as discussed above are attendant to use of electrostatic dissipation materials.
To avoid galvanic corrosion, it is desirable to use for the conductive filler the same metal as that of which the housing is composed. Thus, in the case of an aluminum housing, it would be desirable to use aluminum powder as a filler for the sealant. However, the use of aluminum powder is disadvantageous in that the oxide film on the aluminum particles prevents the passage of electricity due to the high resistivity of the oxide.